DISK PRECOMPENSATION

7/20/2008

Precompensation: A technique used by some early hard drives for writing to the innermost tracks where bit densities are high compared to the rest of the disk. Precompensation slightly advances the end time for strings of identical bits to compensate for the tendancy of magnetic domains written in a densely populated regions to spread out more than they would in less tightly packed areas.

Write precompensation settings were included in the BIOS disk data tables used by early hard drives. The preset settings are carried along in many BIOSes to this day although they are rarely (probably never) used because the disk types in the drive tables are for disks too small for modern applications. It is not clear whether any PC hard disk/controller actually required, or was able to use, user supplied write precompensation values, but it's probable that a few could apply precompensation. Precompensation appears never to have been widely used. Precompensation is also used on some floppy drives where it is specified as a time of few hundred nanoseconds.

In the case of hard drives, it is unclear whether the precompensation value was intended to be a time or a track at which to start applying a fixed precompensation. To further complicate things, when hard drive designers speak of precompensation, they may be referring to a signal amplitude adjustment used to deal with the tendency of disk read/write heads to fly at different distances from the disk on the inner tracks than on the outer tracks.